The present invention relates to a method for unsticking (disengaging) the contacts of a sticking relay whose one contact is arranged locally fixed and whose other contact is arranged at the ferromagnetic relay armature which is attracted with its contact toward the locally fixed contact by the application of an electric voltage to the winding of the relay coil and which drops off again in the de-energized condition of the relay coil with an intact relay whereby the two contacts are again separated from one another.
In electric circuits, load current circuits are frequently switched with the aid of electric relays. However, the disadvantage of these electric relays resides in that the relay contacts at times stick together by reason of the high electric current to be switched, i.e., are welded together by the electric arc occurring during the switchingof the relay. If the relay contacts remain stuck together, the loads arranged in the load current circuit of the relay are frequently destroyed.
It is therefore the object of the present invention to provide a method for the loosening or disengagement of the contacts of a sticking relay, by means of which a destruction of the loads arranged in the load circuit of the relay can be prevented.
The underlying problems are solved according to the present invention in that a sticking together of the relay contacts is determined and in the case of sticking relay contacts, an alternating current (a.c.) voltage or a pulsating d.c. voltage is applied to the winding of the relay coil.
By means of the a.c. voltage or pulsating d.c. voltage applied to the winding of the relay coil when relay contacts stick together, the relay armature is shaken or vibrated by the force exerted thereon by the relay coil, as a result of which the for the most part brittle connecting layer between the relay contacts breaks and the relay again drops off. It is possible by this method to disengage again the sticking relay contacts so rapidly that a destruction of the loads arranged in the load circuit of the relay is prevented.
Preferably, an a.c. voltage or a pulsating d.c. voltage is applied to the winding of the relay coil with a frequency corresponding to the natural (resonant) frequency of the mechanical relay structure. As a result thereof, the relay structure is set into resonant vibrations.
In order to prevent a continuing shaking or vibrating at the relay armature in case that the connecting layer between the relay contacts does not break, the a.c. voltage or the pulsating d.c. voltage is applied advantageously to the winding of the relay coil only for a certain predetermined period of time.
According to a further feature of the method in accordance with the present invention, the development of the load signal to be switched by the relay is compared with the development of the control signal of the relay for the purpose of determining the sticking together of the relay contacts. If the development of the load signal does not follow the development of the control signal of the relay, taking into consideration the shifting time of the relay, then this means that the relay contacts are stuck together.
A circuit arrangement according to the present invention for carrying out the method includes advantageously a comparator circuit controlled at its one input by the control signal of the relay and connected with its other input to the load circuit to be switched by the relay, whose output signal controls a switching device, by means of which in lieu of the control signal of the relay the output signal of a generator producing an a.c. voltage or a pulsating d.c. voltage is adapted to be connected to the relay winding, respectively, to the control input of a relay driver controlling the electric current through the relay winding. In order to prevent a permanent shaking or vibrating of the relay armature when the connecting layer between the sticking relay contacts does not break, a timing circuit is preferably provided between the output of the comparator circuit and the control input of the switching device, which connects-through an output signal of the comparator circuit indicating sticking relay contacts to the control input of the switching device only for a finite predetermined period of time. This circuit arrangement is characterized in particular by its simple construction from a circuit point of view.